1. Field of the Invention
The present invention relates to a printing apparatus and a printing method in which a print medium is printed by allowing a print head to eject ink, and in particular, to a printing apparatus and a printing method which involve sensing of the condition in which ink is ejected through ejection ports.
2. Description of the Related Art
In recent years, ink jet printing apparatuses have been prevailing rapidly, wherein printing is performed by allowing a print head to eject ink droplets onto a print medium. Many ink jet printing apparatus adopt an ink droplet ejection method in which ink is heated to cause film boiling so that the resulting pressure allows ink droplets to fly as an ink droplets ejection method. Unlike an electrophotographic scheme, this method requires no intermediate transfer unit and thus needs few intervening elements in forming images. Hence, the method has the advantage of allowing intended images to be stably obtained.
On the other hand, in the ink jet printing apparatus, inappropriate ink ejections may result from, for example, blockage of ejection ports by dust or thickened ink or coverage of ejection ports with ink droplets. Such an inappropriate ink ejection may be an ejection failure state in which no ink droplets are ejected from ejection ports, a state in which ink droplets are ejected but in which an ejection speed is lower than a predetermined value, resulting in a failure to impact print media, or deviation of impact positions of ejected ink droplets. When such an inappropriate ink ejection is occurred, particularly in an ink jet printing apparatus with a small number of passes, corresponding defective portions concentrate in particular areas of print images. Thus, there are some cases that white or black stripes are generated.
The ink jet printing apparatus with a small number of passes is used by the printing apparatus to perform printing relatively large-sized print media mainly, for example, used for commercial applications such as printing of posters and POP advertisements and industrial applications. Thus, to prevent the above-described image defects (white or black stripes), some printing apparatuses configured to print relatively large-sized print media perform what is called ejection failure compensation printing in which an ejection port located adjacent to an ejection port with an inappropriate ejections is used to carry out compensation printing. Therefore, even when the ejection failure state is detected, forming of high quality image printing is kept.
A technique using a light emission element and a light reception element is known as means for sensing the occurrence position of the ink ejection failure state for the ejection failure compensation printing. In a method for sensing the ink ejection failure state using this sensing means, the light emission element and the light reception element are positioned such that ink droplets pass between these elements. Thus, when ink is ejected, a change in the quantity of light passing between these elements is sensed. In this manner, the occurrence position of the ejection failure state is sensed by sequentially detecting, for each ejection port, whether or not light emitted by the light emission element is blocked by ink droplets.
In the sensing of the ejection failure state, the ejection failure state can be found earlier by increasing the frequency of sensing operations. Then, ejection ports in the ejection failure state can be compensated for. However, since the process of sensing is carried out individually on the respective ejection ports, particularly if the printing apparatus includes a large number of ejection ports like those configured to print large-sized print media as described above, printing needs to be halted for a long time. Thus, when the sensing of the ejection failure state is excessively frequently carried out, the time for which printing is suspended for the sensing of the ejection failure state increases unnecessarily. This may reduce printing throughput.
In view of the above-described circumstances, in order to carry out a process of sensing the ejection failure state at a frequency such that a decrease in throughput is minimized, while maintaining as high image quality as possible, Japanese Patent Laid-Open No. 2007-290352 discloses a method of adjusting the frequency of the ejection failure state sensing process with the accumulated number of ejections focused on. In Japanese Patent Laid-Open No. 2007-290352, the number of ink ejections is counted from the last ejection failure state sensing process. Then, when the ejection number exceeds a predetermined value, the next ejection failure state sensing process is carried out.
By the way, in recent years, even for printing apparatuses configured to print large-sized print media, there has been a demand for high-definition images of a photographic image quality level. For consumer use, ink jet printing apparatuses that use very small droplets have already been provided as instruments that print images with photographic image quality. Such ink jet printing apparatuses configured to print images with the photographic image quality generally eject at most 5 pl of droplets. The amount of the smallest droplets ejected in some recent ink jet printing apparatuses with such a configuration is smaller than 1 pl. The printing apparatus configured to eject very small droplets also includes ejection nozzles allowing formation of dots each corresponding to several dots of the smallest droplets to compensate lowered speed of printing generated by decrease in amount of droplets. Hence, recent ink jet printing apparatuses with the photographic image quality each include a plurality of ejection ports configured to eject different amounts of droplets in which both amounts are equal to or smaller than 5 pl.
The present inventors developed a printing apparatus configured to print large-sized print media and including a plurality of ejection ports configured to eject different amounts of droplets in which both amounts are equal to or smaller than 5 pl. However, the present inventors found that an ejection failure state not observed in the conventional art occurred in this printing apparatus. Furthermore, this new ejection failure state occurred much more frequently than those recognized in the conventional art. Thus, when an attempt is made to compensate for the ejection failure state with the conventional design concept unchanged, achieving both high image quality and a high throughput is very difficult.